It is known that creatine plays a central role in the regulation of energy metabolism and, recently, several groups (Harris et al, 1992; Greenhaff et al, 1993) inducing our own (Ziegenfuss et al, 1994; Boska et al, 1995; Lemon et .al, 1995) have investigated it potential role as an ergogenic aid especially during intense, repeated exercise. In t is project, we are examining the role of creatine as a stimulator of protein synthesis based on previous work (Ingwall et al, 1974; Haussinger et al, 1993). Briefly, recreationally active subjects will ingest 1.0 mg/kg body mass of 15N-glycine as a bolus (0600 h) folIo ed by six, 3 hourly dosages of 0,03 mg!kg. The isotope ingestion will begin 48 h following the previous exercise session. Subjects will remain inactive (other than activities of daily living) throughout 18 h and energy intake will be controlled by providing small meals (6.6 ml/kg of Ensure = 7.0 kcal/kg) every 3 hours. Rates of whole body nitrogen flux, protein synthesis, and degradation will be calculated from total urinary 15 enrichment N using stochastic analysis.